This invention relates to a process for reducing the ammonia content of a gaseous effluent from urea production plants. All industrial urea production plants are based on the following direct synthesis reaction starting from ammonia and carbon dioxide: EQU 2NH.sub.3 +CO.sub.2 .dbd.CO(NH.sub.2).sub.2 +H.sub.2 O
In industrial plants of total recycle type the ammonia, generally liquid, and the carbon dioxide, generally gaseous, are fed in quantities slightly exceeding the stoichiometric values. The difference is due to ammonia and carbon dioxide losses which, either as such or in the form of urea, escape to atmosphere and can pollute the environment.
Said losses can be continuous or discontinuous, liquid or gaseous, the latter possibly containing solid products (urea) or gaseous products (ammonia).
All can result in environmental pollution of a more or less serious extent depending on the amount of loss and the plant position (extent of area population), and is continuously monitored to satisfy local regulations, which vary greatly but in general tend to become increasingly restrictive.
It is therefore important to provide a process which virtually nullifies the ammonia content of a gaseous effluent from industrial urea production plants, so nullifying its pollutant effect.
In said plants, after a reaction stage and a number of recycle stages the aforesaid reaction leads to an aqueous urea solution of about 75 wt % concentration which still contains a small percentage of ammonia which has not been converted into urea.
This solution is concentrated to 96-99.8 wt %, during which concentration a large part of the ammonia is removed from the solution together with the water, and is recovered in the plant. The final solution, practically in the form of molten urea, still contains a small percentage of ammonia, which during the final concentration stage is increased by a further quantity which forms during said final concentration stage by the effect of the reaction: EQU 2CO(NH.sub.2).sub.2 .dbd.NH.sub.2 -CO-NH-CO-NH.sub.2 +NH.sub.3
which results in one mole of biuret plus one mole of ammonia from two moles of urea.
After said final concentration stage, the urea solution (molten urea) still containing ammonia but in a quantity of the order of 0.1 wt % or less is solidified into small beads, the solid product obtained being known as prilled urea if solidification is by a prilling process, or granulated urea if solidification is by a granulating process.
Both said solidification processes consist of bringing the molten urea, divided into droplets in the case of prilling or finely sprayed onto a mass of beads which enlarge in the case of granulation, into contact with an air stream which removes the heat of solidification, dries the product in the case of granulation using 96 wt % molten urea as feed, and finally cools the solid beads to a temperature substantially less than solidification temperature. At the end of the solidification stage the solid product obtained, either prilled or granulated urea, contains a certain quantity of free ammonia (60-200 ppm), the remaining ammonia which was contained in the molten urea having been transferred to the solidification and cooling air and hence fed to atmosphere with pollutant effect.
The ammonia content of this air is very small (80-200 ppm), which explains why it was not a problem up to a short time ago. As pollution problems have generally become more sensitive with consequent more restrictive regulations, it has become necessary to eliminate or at least drastically reduce ammonia emission into the atmosphere.
This problem is not easy to solve because the ammonia quantity contained in the air is relatively small. Washing with water is virtually ineffective because of the low partial pressure of the ammonia in the air.
In existing plants for the wet removal of urea particles contained in this air it has been found that they are totally ineffective against ammonia. In addition the air volume concerned is very large, with the result that the equipment required for its removal would be bulky. In this respect, for a 1000 t/d prilled urea plant, this being an average size, 400,000-500,000 Nm.sup.3 /h of air are required.
The situation is aggravated by the fact that the air is fed to atmosphere at a distance from the ground which can reach 60-80 meters, so that in providing a removal plant it would be necessary to convey this large air quantity to ground level, with further investment and operating cost, this latter because of the considerable pressure drop.
In the case of granulation plants, in which the air throughput to be treated is substantially less than in the case of prilling, plants for removing ammonia from the air already exist. They comprise washing with water made slightly acid by the addition of sulphuric acid.
Their removal efficiency is reasonable, but the resultant water quantity is considerable and contains a small percentage of the corresponding salt, which has to find an easy accommodation and in any event involves operating costs for its concentration.